|Ask the Experts|
June 8, 2021
Oxidation of Solder Balls During Secondary Reflow
Can solder balls oxidize and create opens during a second reflow if the circuit board assemblies were properly cleaned after the first reflow?
|Expert Panel Responses|
So, let's set out a few assumptions on which my answer is predicated:
Hope I am reading this correctly - if the solder ball flowed properly during the 1st reflow, then no it could not oxidize and cause an open. If the solder ball did not flow properly, then you already have a reject condition.
If you are having a test failure and pressing on the BGA changes the test results, then you most likely have solder balls that did not fully collapse/flow.
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
You didn't provide much information along with your question, so I'll have to assume you are referring to the solder balls in the solder paste used during first-side processing.
Those solder balls should have already been formed into solder joints during first-side reflow, so external oxidation should then be irrelevant for second-side processing.
Principal Product Engineer
Benchmark Electronics, Inc.
In general, proper cleaning would remove solder balls from the solder mask. Balls, like the solder joints created during the first reflow probably won't oxidize. You may want to look at the root cause of the solder ball creation.
Printing solder paste on to solder mask is a common cause, stencil design can fix this. Another thing to look at is the temperature ramp up in the first reflow. If it is too fast, solder balls may be formed.
Director of Reclaim Business
Alpha Assembly Solutions
If you are talking about solder spheres on BGA components, then oxidation can and will occur to the metal surfaces during 2nd reflow. This will not normally result in an open circuit assuming the solder joints were properly formed during 1st reflow.
Cleaning no clean flux residues off of the circuit board removes the potential protection from oxidation that the no clean flux itself may provide. Water soluble flux residues are potentially corrosive and must be removed, and typically do not help with oxidation protection as well as no clean fluxes.
I can't understand the question well. Do you mean the solder balls of a BGA or Solder balls generated by the soldering process? My comment to you that the oxidation happens always that the solder or metallic surfaces are in contact with the oxygen, and is accelerates by heat.
Open contacts on balls from BGA, can happens by several causes and reasons but is hard that opens by oxidation by second reflow. Normally the second reflow accentuates the problems that happened on the first reflow or other kind of problems.
Engineering Director / Master IPC Trainer (MIT)
Regarding the question following question, "Can solder balls oxidize and create opens during a second reflow if the circuit board assemblies were properly cleaned after the first reflow?" This question raises more question than a simple answer.
Typically solder balls would not cause this, however what type of solder ball and what type of component. Solder balls in a halo around the solder joint would not, but this also may cause insufficient solder in the solder connection. It depends on the severity of the solder balls. In the case of mid chip solder balls (beads) this would be of greater risk to cause some type of lift.
This would be dependent of board type, solder bead size solder bead location and final geometry of solder connection. Then there is the question regarding cleaning. What chemicals are used in cleaning.
Is it possible that an aqueous cleaning has trapped water under components and the steam generated in second side reflow is causing the lifting? What type of board is used in this assembly? Flex, rigid multilayer, hybrid combination all could play into board movement during reflow and the solder connection could have been compromised during cool down or reheating.
Deck Street Consultants
It is assumed that the second reflow is meant the second side of the Printed Circuit Board. In soldering the intermetallic compounds (IMC) appear as a solid layer on the interface between the tin and copper, commonly referred as intermetallic layer or diffusion zone.
Next to the copper will be the thin layer of Cu3Sn which is further covered with a thicker layer Cu6Sn5. At normal soldering temperatures, these layers are solid, but above 415oC, they begin to dissolve in the molten solder.
Submicron size voids can get developed either at the interface between the IMC and copper land or within the IMC. These voids will get developed, if the solder joints are exposed to elevated temperature such as in high temperature life tests and thermal cycling.
In the particular case, it is the reflow temperature exposure only and duration is very much limited. Moreover, if the second side reflow is carried out immediately after the first side reflow and proper cleaning or the cleaned PC board assembly is kept in nitrogen purged desiccators (due to anticipated delays) before second side reflow, neither oxidation nor open joints can occur.
Vikram Sarabhai Space Centre
I'll assume we're talking about solder spheres on a BGA device. Solder spheres can oxidize to some degree in a 2nd reflow pass but if the solder joint was properly formed in the first reflow, when the solder balls go liquidus in the second reflow the wetting forces should cause the BGA to maintain contact with the PWB. A Head-in-Pillow defect would have occurred in the first reflow pass and wouldn't be exacerbated by the second pass, although you may be seeing it after the 2nd pass. I'd confirm that there is no opens after 1st pass.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Your question didn't provide much information about the assembly, so I'm going to assume you mean an assembly with BGA and the solder balls that forms the solder joint for it. If your process for the first reflow are under control and the solder joints created in it are verified and fulfills the requirements, there should not be any issue in the second reflow. There will be some oxidation but it should not be enough to cause any opens.
I have done a lot of tests with daisy chain units on this scenario, where I have actually gotten quite the opposite result. BGA solder joint that was open after first reflow created a functioning solder joint during the second. However, this is definitively not anything I would count on (and I never tested the strength and reliability on them) - the most important thing is still to have an overall process control and control of the first reflow to get the desired result. To verify it I would do some x-ray analysis and maybe a cross section.
Senior Quality Engineer - PCB Assembly
Please find the Below causes of generate of solder ball during reflow soldering.
There is possibility of Solder ball get oxidised while 2nd pass reflow if amount of flux accumulated with solder ball,
Senior Engineer SMT
Advance Power Display system Ltd Mumbai
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